ConfigMaps

本文由 简悦 SimpRead 转码， 原文地址 kubernetes.io

A ConfigMap is an API object used to store non-confidential data in key-value pairs. Pods can consume ConfigMaps as environment variables, command-line arguments, or as configuration files in a volume.

ConfigMap 是一个API 对象，用于以键值对形式存储非关键性数据。Pod可以以环境变量，命令行参数，或存储卷中的配置文件的形式来消费ConfigMap.

A ConfigMap allows you to decouple environment-specific configuration from your container images, so that your applications are easily portable.

Caution: ConfigMap does not provide secrecy or encryption. If the data you want to store are confidential, use a Secret rather than a ConfigMap, or use additional (third party) tools to keep your data private.

ConfigMap允许用户将根据环境定制的配置文件和容器镜像进行解耦，从而使应用更加便携

注意： ConfigMap本身并不提供任何密钥或加密。如果需要存储关键信息，应该使用Secret而不是ConfigMap，获取使用其他的第三方工具来使数据私密

Motivation 动机

Use a ConfigMap for setting configuration data separately from application code.

使用ConfigMap 来让设置配置信息的过程和应用程序代码分离

For example, imagine that you are developing an application that you can run on your own computer (for development) and in the cloud (to handle real traffic). You write the code to look in an environment variable named DATABASE_HOST. Locally, you set that variable to localhost. In the cloud, you set it to refer to a Kubernetes Service that exposes the database component to your cluster. This lets you fetch a container image running in the cloud and debug the exact same code locally if needed.

A ConfigMap is not designed to hold large chunks of data. The data stored in a ConfigMap cannot exceed 1 MiB. If you need to store settings that are larger than this limit, you may want to consider mounting a volume or use a separate database or file service.

ConfigMap object

A ConfigMap is an API object that lets you store configuration for other objects to use. Unlike most Kubernetes objects that have a spec, a ConfigMap has data and binaryData fields. These fields accept key-value pairs as their values. Both the data field and the binaryData are optional. The data field is designed to contain UTF-8 byte sequences while the binaryData field is designed to contain binary data as base64-encoded strings.

ConfigMap是一个API对象，该对象允许用户存储配置信息供其他对象使用。和其他拥有spec字段的k8s对象不配，ConfigMap由data和binaryData字段。这些字段接收键值对信息作为值。data和binaryData都是可选的。data字段用于包含UTF-8字节序列binaryData字段用于包含二进制数据，例如base64加密的字符串

The name of a ConfigMap must be a valid DNS subdomain name.

ConfigMap的名字必须是有效的DNS子域名

Each key under the data or the binaryData field must consist of alphanumeric characters, -, _ or .. The keys stored in data must not overlap with the keys in the binaryData field.

data或binaryData字段由字母、数字、-,_,.组成。这些存储在data中的关键字不能与binaryData字段中的关键字重复

Starting from v1.19, you can add an immutable field to a ConfigMap definition to create an immutable ConfigMap.

从v1.1.9开始，可以向ConfigMap定义中添加immutable字段来创建一个不可修改的ConfigMap

ConfigMaps and Pods

You can write a Pod spec that refers to a ConfigMap and configures the container(s) in that Pod based on the data in the ConfigMap. The Pod and the ConfigMap must be in the same namespace.

Here’s an example ConfigMap that has some keys with single values, and other keys where the value looks like a fragment of a configuration format.

apiVersion: v1
kind: ConfigMap
metadata:
  name: game-demo
data:
  # property-like keys; each key maps to a simple value
  player_initial_lives: "3"
  ui_properties_file_name: "user-interface.properties"
  # file-like keys
  game.properties: |
    enemy.types=aliens,monsters
    player.maximum-lives=5    
  user-interface.properties: |
    color.good=purple
    color.bad=yellow
    allow.textmode=true

There are four different ways that you can use a ConfigMap to configure a container inside a Pod:

1. Inside a container command and args
2. Environment variables for a container
3. Add a file in read-only volume, for the application to read
4. Write code to run inside the Pod that uses the Kubernetes API to read a ConfigMap、

可以使用以下四种方法来在Pod中对容器进行配置

1. 容器内指定命令和参数
2. 容器内的环境变量
3. 在只读存储卷中添加文件，让应用来读取
4. 在Pod编写代码，使用K8S API来读取配置信息

These different methods lend themselves to different ways of modeling the data being consumed. For the first three methods, the kubelet uses the data from the ConfigMap when it launches container(s) for a Pod.

The fourth method means you have to write code to read the ConfigMap and its data. However, because you’re using the Kubernetes API directly, your application can subscribe to get updates whenever the ConfigMap changes, and react when that happens. By accessing the Kubernetes API directly, this technique also lets you access a ConfigMap in a different namespace.

Here’s an example Pod that uses values from game-demo to configure a Pod:

apiVersion: v1
kind: Pod
metadata:
  name: configmap-demo-pod
spec:
  containers:
    - name: demo
      image: alpine
      command: ["sleep", "3600"]
      env:
        # Define the environment variable
        - name: PLAYER_INITIAL_LIVES # Notice that the case is different here
                                     # from the key name in the ConfigMap.
          valueFrom:
            configMapKeyRef:
              name: game-demo           # The ConfigMap this value comes from.
              key: player_initial_lives # The key to fetch.
        - name: UI_PROPERTIES_FILE_NAME
          valueFrom:
            configMapKeyRef:
              name: game-demo
              key: ui_properties_file_name
      volumeMounts:
      - name: config
        mountPath: "/config"
        readOnly: true
  volumes:
    # You set volumes at the Pod level, then mount them into containers inside that Pod
    - name: config
      configMap:
        # Provide the name of the ConfigMap you want to mount.
        name: game-demo
        # An array of keys from the ConfigMap to create as files
        items:
        - key: "game.properties"
          path: "game.properties"
        - key: "user-interface.properties"
          path: "user-interface.properties"

A ConfigMap doesn’t differentiate between single line property values and multi-line file-like values. What matters is how Pods and other objects consume those values.

For this example, defining a volume and mounting it inside the demo container as /config creates two files, /config/game.properties and /config/user-interface.properties, even though there are four keys in the ConfigMap. This is because the Pod definition specifies an items array in the volumes section. If you omit the items array entirely, every key in the ConfigMap becomes a file with the same name as the key, and you get 4 files.

Using ConfigMaps

ConfigMaps can be mounted as data volumes. ConfigMaps can also be used by other parts of the system, without being directly exposed to the Pod. For example, ConfigMaps can hold data that other parts of the system should use for configuration.

The most common way to use ConfigMaps is to configure settings for containers running in a Pod in the same namespace. You can also use a ConfigMap separately.

For example, you might encounter addons or operators that adjust their behavior based on a ConfigMap.

Using ConfigMaps as files from a Pod

To consume a ConfigMap in a volume in a Pod:

1. Create a ConfigMap or use an existing one. Multiple Pods can reference the same ConfigMap.
2. Modify your Pod definition to add a volume under .spec.volumes[]. Name the volume anything, and have a .spec.volumes[].configMap.name field set to reference your ConfigMap object.
3. Add a .spec.containers[].volumeMounts[] to each container that needs the ConfigMap. Specify .spec.containers[].volumeMounts[].readOnly = true and .spec.containers[].volumeMounts[].mountPath to an unused directory name where you would like the ConfigMap to appear.
4. Modify your image or command line so that the program looks for files in that directory. Each key in the ConfigMap data map becomes the filename under mountPath.

This is an example of a Pod that mounts a ConfigMap in a volume:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: foo
      mountPath: "/etc/foo"
      readOnly: true
  volumes:
  - name: foo
    configMap:
      name: myconfigmap

Each ConfigMap you want to use needs to be referred to in .spec.volumes.

If there are multiple containers in the Pod, then each container needs its own volumeMounts block, but only one .spec.volumes is needed per ConfigMap.

Mounted ConfigMaps are updated automatically

When a ConfigMap currently consumed in a volume is updated, projected keys are eventually updated as well. The kubelet checks whether the mounted ConfigMap is fresh on every periodic sync. However, the kubelet uses its local cache for getting the current value of the ConfigMap. The type of the cache is configurable using the ConfigMapAndSecretChangeDetectionStrategy field in the KubeletConfiguration struct. A ConfigMap can be either propagated by watch (default), ttl-based, or by redirecting all requests directly to the API server. As a result, the total delay from the moment when the ConfigMap is updated to the moment when new keys are projected to the Pod can be as long as the kubelet sync period + cache propagation delay, where the cache propagation delay depends on the chosen cache type (it equals to watch propagation delay, ttl of cache, or zero correspondingly).

ConfigMaps consumed as environment variables are not updated automatically and require a pod restart.

Immutable ConfigMaps

FEATURE STATE: Kubernetes v1.21 [stable]

The Kubernetes feature Immutable Secrets and ConfigMaps provides an option to set individual Secrets and ConfigMaps as immutable. For clusters that extensively use ConfigMaps (at least tens of thousands of unique ConfigMap to Pod mounts), preventing changes to their data has the following advantages:

• protects you from accidental (or unwanted) updates that could cause applications outages
• improves performance of your cluster by significantly reducing load on kube-apiserver, by closing watches for ConfigMaps marked as immutable.

This feature is controlled by the ImmutableEphemeralVolumes feature gate. You can create an immutable ConfigMap by setting the immutable field to true. For example:

apiVersion: v1
kind: ConfigMap
metadata:
  ...
data:
  ...
immutable: true

Once a ConfigMap is marked as immutable, it is not possible to revert this change nor to mutate the contents of the data or the binaryData field. You can only delete and recreate the ConfigMap. Because existing Pods maintain a mount point to the deleted ConfigMap, it is recommended to recreate these pods.

What’s next

• Read about Secrets.
• Read Configure a Pod to Use a ConfigMap.
• Read The Twelve-Factor App to understand the motivation for separating code from configuration.

Last modified February 18, 2021 at 3:41 PM PST: Add WebhookAdmission reference (108149fa2)